Direct conversion receiver oscillator

Hi there just joined today, I was looking at the diagram of a direct conversion receiver wondering why you need an oscillator. If it oscillates at the exact same frequency as the input signal wouldn't tuner be sufficient? I can understand in a superheterodyne receiver as you need to change the frequency to an intermediate frequency. But what exactly does the local oscillator do in a direct conversion receiver?

Thanks

Pierre

If you multiply a carrier with another CW signal, you obtain the baseband/modulated/coded signal.This is direct conversion technique is called as Zero IF.

You COULD receive without an oscillator but then all your tuned circuits would have to be shifted to change frequency. By mixing the input with an oscillator at the same frequency you produce two new signals, one is Fin + Fosc and the other is Fin - Fosc. The subtractive produce is the one you want because the IF is zero, the additive result can be blocked by a simple audio bandpass filter.

Please be aware that zero IF (direct conversion) is not the same as using a higher IF in a conventional superhet receiver and unless the phase of the incoming carrier and the local oscillator are locked together you will hear beat notes, this is why they are unsuitable for normal AM and FM transmissions and why some types of data modulation won't work either.


Brian.

There are 5 types of receivers invented in a span of about 35 years, between late 19th century and 1930.
On their topologies are based ALL the modern receiver architectures (various flavors).

You can search the net for information on each one:

1. Crystal Radio was invented in the late 19th century.
2. Regenerative Receiver was invented and patented in 1914 by Edwin Armstrong.
3. The Tuned Radio Frequency (TRF) receiver was patented in 1916 by Ernst Alexanderson.
4. The Superheterodyne Receiver was patented in 1918 by Edwin Armstrong.
5. The Direct Conversion Receiver (homodyne or synchrodyne) was developed in 1932 by a team of British engineers.

Thanks for the reply guys. I always thought you could amplify the incoming signal alone and it would work.